Sickle Cell Anemia Jenny Pham Protein Structure & Function Spring 2015

Outline I.Normal Adult Hemoglobin (Hb A) a.Function: Deoxygenated v. Oxygenated II.Mutation in Hb A a.Sickle Cell Hemoglobin (Hb S) III.Sickle Cell Anemia a.Background b.Medical/Biological Nature c.Biochemical Nature (Pathophysiology) d.Treatment

Introduction

Hemoglobin (Hb A) Function: Transport oxygen and other gases throughout the body via erythrocytes (red blood cells, or RBCs). Quaternary Structure: 4 Chains (Domains) – 2 α + 2 β Chains  Each chain is comprised of 8 α helices connected by 7 loop regions.  No β strands. Heme molecule bound within pocket created by the α helices of each chain. PDB: 1HHO Method: X-Ray Diffraction Resolution: 2.10 Å

Ligand: Heme Group Heterocyclic Ring Structure: Protoporphyrin IX – 4 Pyrroles Surrounding the Fe 2+ Ion Location of O 2 binding

Hemoglobin Binding to Oxygen

Close Up of Oxygen Binding to the Heme Group in Hemoglobin Deoxygenated Hemoglobin (Left: 2HHB) Oxygenated Hemoglobin (Right: 1HHO)

Results from a point mutation of a thymine to a adenine at the 17 th nucleotide in the β- globin gene on chromosome 11. – Causing for the 6 th amino acid on the β- chains to change from glutamic acid (red) to valine (green). Mutation: Hemoglobin S (Hb S)

Hb A vs. Hb S Hemoglobin S (Sickle) PDB: 2HBS Method: X-Ray Diffraction Resolution: 2.05 Å Normal Adult Hemoglobin PDB: 2HHB Method: X-Ray Diffraction Resolution: 1.74 Å

Point mutation responsible for the distortion of normal RBCs into sickle-shaped RBCs. – Normal RBCs: pliable, biconcave discs – Sickled RBCs: stiff, sticky Deoxygenated Hb S causes for hemoglobin polymers to form, disrupting the cytoskeleton of the RBC that forms protrusions (sickle shape). – Interruption of the membrane attachment to the cytoskeleton leads to the exposure of negatively charged glycolipids, resulting in the blockage of blood circulation.

It’s an autosomal recessive disorder that is characterized by the homozygous inheritance of the β s allele (HbSS). – Must inherit sickle cell trait from both parents. Most common type of sickle cell disease. Sickle Cell Anemia (SCA)

Most Common Genetic Disorder in US – Affecting 70,000 – 100,000 Individuals – Primarily African Americans = 1 in 500 Common in People from Africa, South and Central America, Caribbean Islands, Mediterranean Islands, India, and Saudi Arabia – Distribution of HbS allele correlated to the distribution of malaria – parasites that thrive in normal RBCs cannot survive in sickled RBCs. Prevalence for SCA

SCA is present at birth but does not manifest until 4 months after birth. Early diagnosis at birth can lengthen lifespan with blood test. – Using amniotic fluid or placenta tissue can be used to diagnose SCA before birth. Signs & Symptoms Vary – can range from mild to severe – Linked to Anemia and Pain Numerous Complications – Examples: Increased Risk of Infections, Splenic Crisis, Acute Chest Syndrome, Multiple Organ Failure Medical/Biological Nature of SCA

Biochemical Nature of SCA

Common Treatments Hydroxycarbamide (Hydroxyurea) Blood Transfusions Allogeneic HSCT – only cure for SCA

Emerging Treatments

References 1.Rees, D., Williams, T., & Gladwin, M. (2010). Sickle-cell disease. The Lancet, 376(9757), Gong, L., Parikh, S., Rosenthal, P., & Greenhouse, B. (2013). Biochemical and immunological mechanisms by which sickle cell trait protects against malaria. Malaria Journal, 12(317). 3.Bender MA, Douthitt Seibel G. Sickle Cell Disease Sep 15 [Updated 2014 Oct 23]. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; Ashley-Koch, A., Yang, Q., & Olney, R. S. (2000). Sickle Hemoglobin (Hb S) Allele and Sickle Cell Disease: A HuGE Review. American Journal of Epidemiology, 151(9), Frenette, P., & Atweh, G. (2007). Sickle Cell Disease: Old Discoveries, New Concepts, and Future Promise. Journal of Clinical Investigation, 117(4), Chakravorty, S., & Williams, T. (2014). Sickle cell disease: A neglected chronic disease of increasing global health importance. Archives of Disease in Childhood, 100(1), George, S., Mitchell, E., Mitchell, N., Eason, T., Fuh, B., & von der Embse, N. (2014). Sickle Cell Disease. Science Scope, 38(4), Harrington, D., Adachi, K., & Royer, W. (1997). The high resolution crystal structure of deoxyhemoglobin S. Journal of Molecular Biology, 272(3), Dutta, S., & Goodsell, D. (2003). Hemoglobin. Retrieved March 1, 2015, from The high resolution crystal structure of deoxyhemoglobin S. (1997). Retrieved March 1, 2015, from 11.The crystal structure of human deoxyhaemoglobin at 1.74 A resolution. (1984). Retrieved March 1, 2015, from Structure of human oxyhaemoglobin at 2.1 A resolution. (1983). Retrieved March 1, 2015, from